Methods for producing aldehydes by the hydroformylation of an olefinically unsaturated organic compound with carbon monoxide and hydrogen (more commonly referred to as synthesis or syn gas) in the presence of a rhodium-phosphorus complex catalyst and free phosphorus ligand are well known in the art as seen; e.g., by the basic low pressure oxo hydroformylation process of U.S. Pat. No. 3,527,809 and the rhodium-catalyzed gas and liquid recycle hydroformylation processes of U.S. Pat. Nos. 4,148,830; 4,247,486 and 4,593,127. The resultant aldehyde products are mixtures of normal (straight chain) and iso (branched chain) aldehydes corresponding to the olefin starting material and result from adding a formyl group (--CHO) at one of the carbon atoms of an ethylenic group (e.g. --CH.dbd.CH.sub.2) of the olefin. For instance, the hydroformylation of propylene produces n-butyraldehyde [CH.sub.3 CH.sub.2 CH.sub.2 CHO] and iso-butyraldehyde [CH.sub.3 CH(CHO)CH.sub.3 ]. In general such hydroformylation processes are preferably designed to produce aldehyde products rich in the normal (straight chain) isomer.
Moreover as taught in U.S. Pat. Nos. 4,148,830 and 4,247,486 such continuous hydroformylation processes inherently produce high boiling liquid aldehyde condensation by-products, e.g. dimers, trimers and tetramers, which may serve as a solvent for the hydroformylation process, as well as other liquid heavies. Thus a small amount of such higher boilers is always invariably contained in the crude aldehyde product mixture obtained even after separating the initial aldehyde product from its lights (e.g. carbon monoxide, hydrogen, unreacted alkylene, alkane by-product, etc.) as in the case of a continuous gas recycle hydroformylation process or after separating the initial aldehyde product from its lights and catalyst containing solution as in the case of a continuous liquid recycle hydroformylation process. Indeed even after separating the lower boiling, branched chain aldehyde from its higher boiling normal straight chain aldehyde counterpart in order to obtain purified branched chain aldehyde (e.g. iso-butyraldehyde) and leave the straight chain aldehyde (e.g. n-butyraldehyde), the normal aldehyde product may still contain a higher amount of such organic heavies than desired for its eventual end-use.
Accordingly, heretofore, it has been the conventional procedure in the art to refine and separate the branched-chain aldehyde product from the straight chain aldehyde product of such crude aldehyde product mixtures resulting from such conventional continuous rhodium catalyzed hydroformylation processes by a two step distillation procedure that involves the use of two separate distillation columns. For example, purified branched chain aldehyde (e.g. iso-butyraldehyde) is first separated from the crude aldehyde product mixture via distillation in an initial distillation column and then the remaining normal (straight chain) aldehyde (e.g. n-butyraldehyde) is further refined or purified from any remaining higher boiling by-products by a second distillation carried out in a second distillation column.
However, there are two major penalties associated with commercially refining the crude aldehyde product mixture via such a dual distillation procedure. The first is the very high energy cost required to operate such dual distillation procedures on a commercial level. Secondly, a significant amount of aldehyde is lost due to in situ conversion into such heavies during such distillation procedures because of the high temperatures employed to recover as much straight chain aldehyde from said organic heavies as possible. Indeed, it has been estimated that as much as 1 to 2 percent by weight or more of straight chain aldehyde may be lost by its own in situ conversion to heavies and such is clearly a significant amount in any commercial operation, such as the above discussed hydroformylation operations, that may produce hundreds of millions of pounds of aldehyde per year.
In a previous commercial operation conducted more than a year prior to the filing of this application at a plant in the United States, owned and operated by assignee, applicant experimented with employing a single distillation column, wherein purified branched chain iso-butyraldehyde was obtained by distilling same overhead and essentially all of the straight chain n-butyraldehyde was collected as a distilled gas from a lower side vent off of the same distillation column. However, as in the case with conventional two stage distillation procedures that involve two distillation columns, the distillation temperature required to obtain essentially all of the n-butyraldehyde off the side vent of the single distillation column was essentially the same high distillation temperature (e.g. about 115.degree. C. to about 140.degree. C.) conventionally employed in distilling n-butyraldehyde from organic heavies in a second distillation column, thus causing essentially the same type of detrimental loss of aldehyde due to in situ heavies formation as normally occurs with a second distillation column.
It has now been discovered that it is not necessary to employ such high distillation temperatures in order to concurrently separate and obtain both purified branched chain aldehyde and purified straight chain aldehyde from a crude aldehyde product mixture using a single distillation column. Thus such drawbacks associated with heretofore conventional distillation refining of crude aldehyde product mixtures may be overcome or at least greatly minimized by the process of this invention and explained more fully below.